The generic equilibrium constant expression for a weak acid is written as follows.
If the acid is strong enough to ignore the dissociation of water, the H3O+ ion and A- ion concentrations in this solution are about equal.
[H3O+] [A-]
Substituting this information into the acid-dissociation equilibrium constant expression gives the following result.
The concentration of the HA molecules at equilibrium is equal to the initial concentration of the acid minus the amount that dissociates: C.
If C is small compared with the initial concentration of the acid, we get the following approximate equation.
Rearranging this equation and taking the square root of both sides gives the following result.
